Print Biology Online http www biology online org kb print php aid 12 Genetics and Evolution Meiosis The Genetics of Reproduction As mentioned in previous pages the genetic information found in DNA is essential in creating all the characteristics of an organism This remains the case when passing genetic information to offspring that can occur via a process called meiosis where four haploid cells are created from their diploid parent cell For a species to survive and genetic information to be preserved and passed on reproduction must occur This can be done by passing on the information found in the chromosomes via the gametes that are created in meiosis Chromosome Complement Humans are diploid creatures meaning that each of the chromosomes in our body are paired up with another Haploid cells possess only one set of a chromosome For example a diploid human cell possesses 46 chromosomes and a gamete created by a human is haploid possesses 23 chromosomes Tetraploid organisms possess more than 3 sets of a particular chromosome Reproduction Reproduction occurs in humans with the fusion of two haploid cells gametes that create a zygote The nuclei of both these cells fuse bringing together half the genetic information from the parents into one new cell that is now genetically different from both its parents This increases genetic diversity as half of the genetic content from each of the parents brings about unique offspring which possesses a unique genome presenting unique characteristics Meiosis as a process can increase genetic variation in many ways explained soon The Process of Meiosis The process of meiosis essentially involves two cycles of division involving a gamete mother cell diploid cell dividing and then dividing again to form 4 haploid cells These can be subdivided into four distinct phases which are a continuous process 1st Division Prophase Homologous chromosomes in the nucleus begin to pair up with one another and then split into chromatids one half of a chromosome where crossing over can occur Crossing offer can increase genetic variation Metaphase Chromosomes line up at the equator of the cell where the sequence of the chromosomes lined up is at random through chance increasing genetic variation via independent assortment Anaphase The homologous chromosomes move to opposing poles from the equator Telophase A new nuclei forms near each pole alongside its new chromosome compliment At this stage two haploid cells have been created from the original diploid cell of the parent 2nd Division Prophase II The nuclear membrane disappears and the second meiotic division is initiated Metaphase II Pairs of chromatids line up at the equator Anaphase II Each of these chromatid pairs move away from the equator to the poles via spindle fibres Telophase II Four new haploid gametes are created that will fuse with the gametes of the opposite sex to create a zygote Overall this process of meiosis creates gametes to pass genetic information from parents to offspring continuing the family tree and the species as a whole Each of these gametes possess unique genetic information due to situations in meiosis where genetic diversity is increased all of which is elaborated upon on the next page Independent Assortment and Crossing Over The previous page investigates the process of meiosis where 4 haploid gametes are created from the parent cell Half the genetic information from a parent is present in these haploids which fuse with gametes of the opposite sex to create a zygote with a complete chromosome compliment that 1 of 14 4 11 14 8 51 PM Print Biology Online http www biology online org kb print php aid 12 will create offspring after prolonged growth The process of meiosis increases genetic diversity in a species The sex organs which produce the haploid gametes are the site of many occurrences where genetic information is exchanged or manipulated Independent Assortment of Chromosomes Alleles for a particular phenotype determine what characteristic an organism will express as with the following example where Chromosome 1 contains an allele for blonde hair Chromosome 2 contains an allele for brown hair Chromosome 3 contains an allele for blue eyes Chromosome 4 contains an allele for brown eyes The top assortment to the left produces 2 blonde hair blue eyes gametes while the below produces 2 brown hair brown eyes gametes The top assortment on the right produces 2 blonde hair brown eyes gametes while the below produces 2 brown hair blue eyes gametes The above indicates that even though the two homologous chromosomes contain the same genetic information the assortment of the chromosomes the order they lie in can determine what genetic information is present in each of the 4 gametes produced With 23 chromosomes in a human gamete their are 223 combinations 8388608 combinations Crossing Over During meiosis when homologous chromosomes are paired together there are points along the chromosomes that make contact with the other pair This point of contact is deemed the chiasmata and can allow the exchange of genetic information between chromosomes This further increases genetic variation There are also many other ways in which genetic variation is increased in a species gene pool all of which are described in the following pages The next page investigates the work of Gregor Mendel an Austrian monk famous for his work involving monohybrid and dihybrid crossing alongside the continuation into looking at genetic diversity through meiosis and genetics in general Crossing Over and Genetic Diversity Gregor Mendel an Austrian monk is most famous in this field for his study of the phenotype of pea plants including the shape of the peas on the pea plants Gregor Mendel s Work Mendel s goal was to have a firm scientific basis on the relationship of genetic information passed on from parents to offspring In light of this he focused on how plant offspring acquired the phenotype of their seeds In this example there are two choices round and wrinkled seeds The plants that were used in the experiment had to be true breeding i e those plants with round seeds must have had parents with round seeds who in turn had parents producing round seeds etc This is done to increase the accuracy of results After successfully producing two generations from these true breeding plants the following was evident The first generation of plants produced all had a round seed phenotype When these first generation plants were crossed a ratio of 3
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